The present invention relates to a control system for a tandem rolling mill for rolling a steel strip or the like.
A tandem rolling equipment comprises a tandem rolling mill in which rolls are rotated at each of a plurality of mill stands disposed successively to feed and roll a strip (rolling material). In the tandem rolling mill for performing such rolling, a feeding shape restricting mechanism called looper is disposed between the mill stands, for such a control that the strip maintains a loop (curve) with a predetermined length between the stands and maintains a desired tension, whereby quality of thickness of strip and width of strip is secured and stable operation is provided.
FIG. 1 is a general constitutional view of a major part of a tandem rolling mill, and FIG. 2 is a block diagram of a control system for a tandem rolling mill using a first prior art described in xe2x80x9cApplication of H∞ Control to Actual Plantxe2x80x9d (The Society of Instrument and Control Engineers) pp. 70-73. In FIG. 1, numeral 1 denotes a tandem rolling mill, 21 denotes a strip which is a rolling material, 22 denotes a former-stage mill stand, 23 denotes a latter-stage mill stand, 24 denotes a mill motor, 25 denotes a looper, 26 denotes a looper motor, 27 denotes a tension detector for detecting the tension of the strip 21, and numeral 28 denotes a looper angle detector for detecting the rotating angle and rotating speed of the looper 25. The control system in FIG. 1 is in a general representation without any symbol, because its constitution differs according to the contents of the applied technique, namely, whether the prior art or an embodiment of the present invention. In addition, the tension detector 27 and the looper angle detector 28 are shown within the box of dotted line indicating the tandem rolling mill 1 for easy recognition, but in actual classification, they belong to the components of the control system. In FIG. 2, numeral 20 denotes a control system according to the prior art, 2 denotes a mill speed controller, 3 denotes a looper torque controller, 5 denotes a tension setting torque arithmetic unit, and 6 denotes a looper angle controller.
Next, action or operation will be described.
In the tandem rolling mill 1, rolls are driven to rotate by the mill motor 24 while making a draft at each of the mill stands 22, 23, here the former-stage mill stand 22, and the strip 21 is rolled by feeding out the strip 21. The looper 25 driven by the looper motor 26 as well as attendant mechanisms is disposed between the mill stands 22 and 23, and the looper 25 is brought into contact with the strip 21 drivingly fed by the mill motor 24, whereby the feeding shape of the strip 21 is restricted. On the other hand, in the control system 20, the mill speed controller 2 controls so as to make the speed of the mill motor 24 coincide with a mill speed command vr, and the looper torque controller 3 controls so as to make the torque of the looper motor 26 coincide with a torque command qr. Namely, the control system 20 appropriately calculates the mill speed command vr and the looper torque command qr, and controls so that the strip 21 forms a predetermined loop (curve) between the mill stands, namely, the angle of the looper 25 has a predetermined value, while giving a predetermined tension to the strip 21.
While a constitution in which the mill speed of the former-stage mill stand 22 is controlled in order to control the tension "sgr" of the strip 21 between the mill stands 22 and 23 and the angle xcex8 of the looper 25 has been presented as an example in the above description, the object of control of the mill speed is not limited to the former-stage mill stand 22, and may be the latter-stage mill stand 23.
The first prior art applied to the control system 20 is a system which has been used most generally for a long time, and, although it has not a specially excellent tension control performance, it promises a simple and easily usable control system. The control system 20 is externally supplied with a tension command "sgr"r and a looper angle command xcex8r, and is supplied with a looper angle xcex8 detected by the looper angle detector 28. The tension setting torque arithmetic unit 5, in the condition where the tension "sgr" of the strip 21 is steadily conformed to the tension command "sgr"r, based on the tension command "sgr"r, calculates in a feed-forward manner a torque of the looper motor 26 for the looper 25 to support the strip 21, and outputs the calculated result as a tension setting torque qs. The tension setting torque qs is inputted as a torque command qr to the looper torque controller 3, which performs such a control as to make the torque of the looper motor 26 coincide with the torque command qr. The looper angle controller 6 is supplied with an angle deviation xcex8e which is the difference between the looper angle command xcex8r and the looper angle xcex8, and calculates the sum of a signal obtained by multiplying the angle deviation xcex8e by an angle proportional gain Cp and a signal obtained by integrating the angle deviation xcex8e and multiplying the integrated value by an angle integral gain Ci. Namely, a PI (proportional integral) arithmetic operation is conducted so that the looper angle xcex8 does not have a steady deviation, and outputs the result of the arithmetic operation as a mill speed command vr. The mill speed controller 2 performs such a control as to make the mill speed coincide with the mill speed command vr.
Thus, the control system 20 generates a tension setting torque qs such that the strip 21 with a tension "sgr" conforming to the tension command "sgr"e is steadily supported by the looper 25 in a balanced manner; in addition, under variations in the speed of the strip 21 generated by variations in the draft at the former-stage mill stand 22 and the latter-stage mill stand 23 or the like, the control system 20 corrects the mill speed command vr so that the looper angle xcex8 coincides with the looper angle command xcex8r, namely, the length of the loop between the former-stage mill stand 22 and the latter-stage mill stand 23 becomes constant. In this manner, the looper angle xcex8 and the tension "sgr" are controlled with the looper angle command xcex8r and the tension command "sgr"r as target values. Furthermore, the control system 20 in this type does not necessarily need the tension detector 27, and feed-back control is conducted by only the looper angle controller 6, so that operation can be continued by only applying easy adjustments to a one-loop control system based on the looper angle controller 6.
However, the simplicity of a control system and the quality of control performance are opposed to each other in many cases. The characteristics of the tandem rolling mill 1 are basically governed by resonance characteristics of a spring inertia system consisting of the elasticity of the strip 21 and the inertia of the looper 25; therefore, the prior art control system 20 in which the tension "sgr" is not feedback controlled has the problem that the control system becomes instable when the gain of the looper angle controller 6 is raised, and it is difficult to control the tension "sgr" and the angle xcex8 with high accuracy.
Next, FIG. 3 shows the constitution of a control system for a tandem rolling mill according to a second prior art described in xe2x80x9cApplication of H∞ Control to Actual Plantxe2x80x9d (The Society of Instrument and Control Engineers) pp. 77-79. The constitution of the tandem rolling mill 1 which is the object of control is as shown in FIG. 1. Numeral 30 denotes a control system according to the second prior art, 201 denotes a looper angle controller, 202 denotes a tension controller, 203 denotes a noninterference controller having coefficient units H12 and H21, and 204 denotes a looper speed controller. The second prior art is characterized by noninterference control, and a large difference from the first prior art lies in that, based on the tension "sgr" detected by the tension detector 27, the mill motor 24 controls the tension "sgr" while the looper motor 26 controls the looper angle xcex8 in a separate manner.
Next, the action or operation of the control system 30 will be described. First, the looper angle controller 201 supplied with a looper angle deviation xcex8e which is the deviation between a looper angle command xcex8r and the looper angle xcex8 outputs a signal obtained by PI arithmetic operation such that the looper angle xcex8 does not have a steady deviation. On the other hand, the tension controller 202 supplied with a tension deviation "sgr"e which is the deviation between a tension command "sgr"r and the tension "sgr" outputs a signal obtained by PI arithmetic operation such that the tension "sgr" does not have a steady deviation. The noninterference controller 203 inputs a sum signal of a signal obtained by multiplying the output of the tension controller 202 by xe2x88x921 and a signal obtained by multiplying the output of the looper angle controller 201 by a constant h12 at the coefficient unit H12 to the mill speed controller 2 as a mill speed command vr, and inputs a sum signal of the output of the looper angle controller 201 and a signal obtained by multiplying the output of the tension controller 202 by a constant h21 at the coefficient unit H21 to the looper speed controller 204 as a looper speed command xcfx89r. The looper speed controller 204 calculates a looper torque command qr so as to make the looper speed xcfx89 detected by the looper angle detector 28 coincide with a looper speed command xcfx89r, and inputs the looper torque command qr to the looper torque controller 3, which controls so as to make the torque generated at the looper motor 26 coincide with the lopper torque command qr.
Here, the looper speed controller 204 is composed of a computer different from those of the looper angle controller 201, the tension controller 202 and the noninterference controller 203, and is ordinarily composed of a computer in a motor drive device similarly to the looper torque controller 3. Besides, the arithmetic operation by the computer in the motor drive device is performed with a sampling period at a higher speed than the arithmetic operations by the looper angle controller 201, the tension controller 202 and the noninterference controller 203. The looper speed controller 204 performs a control including an integral feedback so that the looper speed xcfx89 conforms to the looper speed command xcfx89r even when a steady torque is externally exerted on the looper motor 26, and, for example, calculates the looper speed command qr by a PI arithmetic operation with the deviation between the looper speed command or and the looper speed xcfx89 as an input.
The control system 30 using the second prior art is so configured that the tension "sgr" is controlled by the mill motor 24 while the looper angle xcex8 is controlled by the looper motor 26, and performs such actions that a steady value of the tension "sgr" and a steady value of the looper angle xcex8 are controlled respectively by the mill motor 24 and the looper motor 26. Therefore, there is a characteristic feature that a signal component obtained by integrating the tension deviation "sgr"e is added to the mill speed command vr by the action of the tension controller 202 performing a PI arithmetic operation, and a signal component obtained by integrating the looper angle deviation xcex8e is added to the looper torque command qr by the actions of the looper angle controller 201 and the looper speed controller 204. Incidentally, even where the looper angle controller 201 does not perform the PI arithmetic operation but performs only a proportional arithmetic operation, if the looper speed controller 204 performs the PI arithmetic operation, a proportional component of the looper angle deviation xcex8e is added to the looper speed command xcfx89r, and, further, a component obtained by integrating the looper speed command xcfx89r by the PI arithmetic operation of the looper speed controller 203 is added to the looper torque command qr, so that a signal component obtained by integrating the looper angle deviation xcex8e is added to the looper torque command qr, and the steady value of the looper angle xcex8 is controlled by the looper motor 26, in the same manner as above.
In addition, while the noninterference controller 203 is used for obviating the mutual interference between the control of the tension "sgr" by the mill motor 24 and the control of the looper angle xcex8 by the looper motor 26, a signal component obtained by integrating the tension deviation "sgr"e is added to the mill speed command vr, and a signal component obtained by integrating the looper angle deviation xcex8e is added to the looper torque command qr, as described above, whereby the steady value of the tension "sgr" is controlled by the mill motor 24, and the steady value of the looper angle xcex8 is controlled by the looper motor 26, in the same manner as above.
Since the control system 30 according to the second prior art acts in the manner described above, the steady value of the looper angle xcex8 is controlled by the looper motor 26 irrespectively of the length of the loop of the strip 21 between the mill stands. Therefore, where proportional gains and integral gains of both of the looper angle controller 201 and the tension controller 202 are not appropriately set in a stroke, there arises the problem that the looper 25 might be separated from the strip 21, and adjustments at the time of starting operation, particularly, are difficult.
In addition, although the noninterference controller 203 is used, perfect noninterference between the control of the tension "sgr" and the control of the looper angle xcex8 is impossible. In particular where control gains of the tension controller 202 and the looper angle controller 201 are small, the control of the tension "sgr" and the control of the looper angle xcex8 may interfere with each other to make the control system instable, and it is difficult to adjust the control system in view of this problem. Further, where control gains of the tension controller 202 and the looper angle controller 201 are set sufficiently large, the looper angle xcex8 is fixed by the action of the looper angle controller 201, so that the control of the tension "sgr" is conducted by only the mill motor 24; thus, the looper 25 cannot be actively utilized for the control of the tension "sgr", and, therefore, it is impossible to enhance the accuracy of tension control.
Next, FIG. 4 shows the constitution of a control system according to a third prior art described in The Institute of Electrical Engineers Papers C, Vol. 116, No. 10, pp. 1111-1118, and Japanese Patent Laid-open No. 8-155522 (1996). In FIG. 4, the same symbols as those in FIG. 2 denote the same portions, and the description thereof is omitted. Symbol 40 denotes a control system according to the third prior art, and 205 denotes a multi-variable proportional controller. The third prior art is the first prior art plus the multi-variable proportional controller 205.
Next, the action of the control system 40 will be described. First, in the same manner as the control system 20 according to the first prior art, a looper angle controller 6 is supplied with a looper angle deviation xcex8e and outputs a signal obtained by a PI arithmetic operation, and a tension setting torque arithmetic unit 5 calculates a tension setting torque qs based on a tension command "sgr"r. The multi-variable proportional controller 205 is supplied with a variational tension xcex94a which is a variation of tension "sgr" on the basis of a tension command "sgr"r and with a looper speed xcfx89, and outputs a sum signal vh of a signal obtained by multiplying the variational tension xcex94"sgr" by a set constant h22 and a signal obtained by multiplying the looper speed xcfx89 by a set constant h21, and a sum signal qh of a signal obtained by multiplying the variational tension xcex94"sgr" by a set constant h12 and a signal obtained by multiplying the looper speed xcfx89 by a set constant h11. Subsequently, a sum signal of the output of the looper angle controller 6 and a signal obtained by multiplying the output vh of the multi-variable proportional controller 205 by xe2x88x921 is inputted as a mill speed command vr to a mill speed controller 2, and a sum signal of the tension setting torque qs and a signal obtained by multiplying the output qh of the multi-variable proportional controller 205 by xe2x88x921 is inputted as a looper torque command qr to the looper torque controller 3.
Here, in the control system 40, the proportional gain from the tension "sgr" to the looper torque qr is h12 as described above, and determination of the proportional gain h12 is conducted so that the looper angle xcex8 does not diverge to infinity when the multi-variable proportional controller 205 is operated without operating the looper angle controller 6, namely, on the condition that the variation of the tension "sgr" of the strip 21 in the tandem rolling mill 1 cancels the variation of torque given to the looper 25. Therefore, h12 has a negative sign, and acts so as to increase the looper torque command qr as the tension "sgr" increases. The proportional gain h12 is so set that the variation of the looper angle xcex8 is smaller than the variation of the tension "sgr" under the effect thereof.
As is clear also from the block arrangement, the control system 40 has a structure obtained by only adding the multi-variable proportional controller 205 composed of four proportional gain elements to the first prior art capable of operating the rolling equipment most easily, namely, a one-loop control by use of the looper angle controller 6. Therefore, it suffices to adjust by adding the gains one by one to the one-loop control by the looper angle controller 6, and the adjustment is easier than that in the second prior art, which can be well evaluated.
However, when all the above arithmetic operations are conducted with the same sampling period, the arithmetic operations take time so that the sampling period is long (ordinarily several tens of msec). The long sampling period means that dead time is not negligible, the response characteristics of the control system as a whole are lowered, and, the accuracy of control of the tension "sgr" cannot be sufficiently enhanced accordingly. In addition, the proportional gain h12 from the tension "sgr" to the looper torque command qr has a sign so selected that the looper torque command qr increases as the tension "sgr" increases, and, therefore, actions are so set as not to increase the looper angle xcex8 as much as possible under the variation of the tension "sgr". As a result, the looper 25 cannot be actively utilized for a control for suppressing the variation of the tension "sgr", and the control of the tension "sgr" is conducted by only the mill motor 24, so that the accuracy of control of the tension "sgr" has a limitation, and it is difficult to sufficiently enhance the accuracy.
Thus, the first to third prior arts have the problem that the adjustment of the control system is difficult or that the adjustment is easy but it is difficult to sufficiently enhance the accuracy of control.
The present invention is for solving the above-mentioned problems, and it is an object of the invention to realize a tension control with easy adjustment and high accuracy.
A control system for a tandem rolling mill according to the present invention is applied to a tandem rolling mill for continuously rolling a rolling material by bringing a looper driven to rotate by a looper motor into contact with the rolling material drivingly fed by a mill motor so as to restrict the feeding shape of the rolling material, and includes a looper torque controller provided with a torque command for controlling the torque of the looper motor, and a mill speed controller provided with a mill speed command for controlling the speed of the mill motor, wherein the control system comprises a looper angle controller for performing a control arithmetic operation on a looper angle deviation which is a deviation of looper angle from an externally inputted looper angle command and giving the result of the arithmetic operation to the mill speed controller as a mill speed command; and a looper speed controller operating at an arithmetic operation speed higher than that of the looper angle controller, for performing a control arithmetic operation on a looper speed deviation which is a deviation of looper speed from an externally inputted looper speed command, and giving the result of the arithmetic operation to the looper torque controller as a torque command utterly irrelevant to the output of the looper angle controller.
With this configuration, the result of arithmetic operation obtained by a control arithmetic operation performed on a looper speed deviation by the looper speed controller is supplied to the looper torque controller as a torque command utterly irrelevant to the output of the looper angle controller, and a component obtained by integrating a looper angle deviation is not contained in the torque command outputted from the looper speed controller, so that a steady value of looper angle is not controlled separately from a steady value of tension, whereby a control with high accuracy can be made by a simple adjustment on the basis of a one-loop control by the looper angle controller. Further, a variation in looper speed is compensated for by the looper speed controller at an arithmetic operation speed higher than that of the looper angle controller, so that it is possible to reduce the dead time of the control system arising from the length of arithmetic operation period, to control the speed of the looper motor with a sufficient instant response characteristic, and to largely enhance the quality of rolling material tension control, looper angle control and looper speed control.
The control system for the tandem rolling mill according to the present invention is such that the looper speed controller comprises a looper speed proportional controller for proportionally multiplying the looper speed deviation and adding the product to a looper torque command calculated based on a tension command that is a tension target value for the rolling material.
By this, enlarging the proportional gain of the looper speed proportional controller means a proportional compensation of variations in the looper speed by a looper torque command, and the variations in the looper speed due to variations in the torque externally exerted on the looper and the looper motor is controlled. In addition, the feedback control of the looper speed in the looper speed controller is a proportional control, which is irrelevant to integral control containing a time term, so that the looper speed will not become 0 to cause stoppage unless the tension deviation is not 0. Therefore, the variation of the looper angle is not excessively suppressed, as contrasted to the case where the looper speed controller performs integral control also, so that there is little risk that the looper is separated from the rolling material to generate a substantial uncontrollable period. Thus, a stable operation with high quality can be achieved.
The control system for the tandem rolling mill according to the present invention comprises a tension intersection proportional controller for proportionally multiplying a tension deviation which is a deviation of tension from a tension command, and adding the product to the looper speed command.
By this, the tension intersection proportional controller performs such an arithmetic operation as to decrease the looper torque command as the tension increases, and the variation in the tension can be compensated for by actively operating the looper with fast response. As compared with the prior art not using the looper speed controller, the variation direction of the looper angle relative to the tension is the same; however, the variation of tension can be suppressed largely as much as the looper is swiftly operated by the looper speed controller as if the inertia of the looper were reduced. Further, the variation width of the looper angle will not be excessively enlarged because it is suppressed by the operation of the looper angle controller. In addition, since the response of the looper speed relative to the variation in tension is increased, it is possible to set the response characteristic of the looper angle controller to be high, to enhance the response characteristic of the control system as a whole, and to secure a stable operation through enhancing the accuracy of the looper angle control and tension control.
The control system for the tandem rolling mill according to the present invention comprises a tension proportional controller for proportionally multiplying a tension deviation that is a deviation of tension from a tension command, and causing the product to be a subtraction input for said mill speed command.
By this, tension variation can be proportionally compensated for with a mill speed command according to the proportional gain of the tension proportional controller; as a result of the proportional compensation, the tension variation is suppressed, whereby a vibration damping effect of the control system is enhanced, and a stable operation can be more rigidly secured.
The control system for the tandem rolling mill according to the present invention is such that the tension proportional controller comprises a computer operating at an arithmetic operation speed higher than that of the looper angle controller.
By this, of the governing elements of the innermost control loop in the entire control system, the looper speed controller as well as the tension proportional controller can be made to perform high-speed arithmetic operations. Since the innermost control loop in the entire control system required of the fastest response is enhanced in speed, the response of the entire control system can be enhanced in speed, the dead time of the control system due to the length of arithmetic operation periods can be reduced. In addition, the looper motor is controlled in speed with a sufficient instant response characteristic, whereby the quality of control of the entire control system can be enhanced.
The control system for the tandem rolling mill according to the present invention comprises a tension integral controller for performing an integrating arithmetic operation on a tension deviation which is a deviation of tension from a tension command, and adding the result of the arithmetic operation to a tension setting torque.
By this, even where an offset error is steadily generated between a tension steadily balanced by a tension setting torque calculated by a tension setting torque arithmetic unit and a steady value of tension detected by a tension detector due to detection and arithmetic operation errors or the like, generation of the steady offset error can be prohibited by the integral arithmetic operation of the tension integral controller, so that a tension control with higher accuracy can be achieved. In addition, since an integrated value of tension deviation is not added to the mill speed command but is added only to the looper torque command, there is no need to add a control loop for adding an integral component of the looper angle deviation to the looper torque command. Furthermore, since compensation of the offset error does not require an instant response characteristic, a tension integral gain may have a small value. Therefore, the fear that the dynamic characteristics of the entire control system would be deteriorated due to the provision of the tension integral controller is needless, and the tension control performance is enhanced by the addition of a minimum number of control loops, whereby stability and quality of operation can be enhanced.
The control system for the tandem rolling mill according to the present invention is such that the looper speed command externally inputted to the looper speed controller is fixed at zero, and a value obtained by multiplying the looper speed by a negative constant is set as a torque command in the looper torque controller.
By this, the looper speed controller does not receive a looper speed command according to tension deviation, and performs a control action under the condition where the looper speed command is replaced with 0 irrespectively of tension. The innermost control loop is only a control loop for feeding back the looper speed to the looper torque command, and a control loop for feeding back the tension to the mill speed command may be absent. Therefore, although a high-speed response characteristic is somewhat poor, quick response to deviations can be secured by an arithmetic operation at a higher speed than the looper angle controller, whereby response of the entire control system can be enhanced in speed, and the accuracy of control of tension can be enhanced.
The control system for the tandem rolling mill according to the present invention comprises a looper speed intersection proportional controller for proportionally multiplying the looper speed, and causing the product to be a subtraction input for the mill speed command.
By this, in the case where it is desired to further reduce the variations in the looper angle, though difficulty of adjustment increases as much as the adjustment gain increases, the effect of actively using the looper for tension control is weakened, the variation of looper angle is further reduced, and the ratio of using the mill motor and the looper motor for tension control is adjusted, whereby optimum control can be realized.
The control system for the tandem rolling mill according to the present invention comprises a looper angle proportional controller for proportionally multiplying the looper angle deviation which is an input to the looper angle controller, and adding the product to the looper speed command which is an input to the looper speed controller.
By this, where a looper angle proportional control gain is set at a small appropriate value, the response of tension deviation is once swung largely to the negative side and can then be stabilized without swinging to the positive side, whereby a transient variation suppressing type tension control can be realized. In addition, since a signal component obtained by integrating looper angle deviation is not added, a steady value of tension produces an effect on a steady value of looper angle in the same manner as before. Since the steady value of the looper angle and the steady value of tension are not controlled separately from each other, the accuracy of control of tension can be enhanced by an easy adjustment of adding proportional control loops one by one to a one-loop control system, in a supplementary manner, on the basis of the looper angle controller.
The control system for the tandem rolling mill according to the present invention is such that the looper speed controller comprises a looper speed proportional controller for proportionally multiplying the looper speed deviation by a looper speed gain, and the looper speed proportional controller is given a looper speed command including a value obtained by dividing by the looper speed gain a tension setting torque calculated based on a tension command which is a tension target value for the rolling material.
By this, a tension command which is a tension target value for the rolling material is not added at the output stage of the looper speed proportional controller, and a looper speed command including a value obtained by dividing a tension command by the looper speed gain is given on the former stage of the looper speed controller, so that, substantially, the looper speed gain and the reciprocal thereof cancel each other and a tension command with a gain of 1 is added to the output of the looper speed proportional controller, thereby contributing to an increase in processing speed of the looper speed controller in which one adding process is omitted. In addition, since the looper speed controller performs only a proportional control, the variations in the looper angle are not excessively suppressed. Further, since the looper speed controller does not perform an arithmetic operation of a time function, it is easy to externally set a steady looper torque command.
The control system for the tandem rolling mill according to the present invention is such that the looper speed controller comprises a looper speed proportional-integral controller for performing a proportional-integral arithmetic operation on the looper speed deviation, and adding the result of the arithmetic operation to a looper torque command calculated based on a tension command which is a tension target value for the rolling material.
By this, the looper speed controller performs an integral action in addition to a proportional action, whereby a value obtained by integrating the tension deviation is included in the looper torque command, and the steady value of the looper torque command can be set at such a value that a steady deviation of tension is not generated.